One dye useful, for example, in marking inks is a salt of 3,3′-dimethylbiphenyl-4,4′-diyl)dihydrazin-2-yl-1-ylidene]bis(4-amino-5-oxo-5,6-dihydronaphthalene-1,3-disulfonate (hereinafter referred to by CAS No. 314-13-6). Its chemical features include multiple functional groups including sulfate, hydrazo, amine and hydroxy (namely phenoxy). The hydroxy is adjacent to amine and to a nitrogen in hydrazo, such that the hydrogen of hydroxy is believed to tautomerize with one or both adjacent electron donor moieties. This tautomerization, which is duplicated at two ends of the molecule, is believed to contribute to the excellent color of CAS No. 314-13-6. As such, this compound may be used as a dye, for example, in a marking ink, used, for example, to mark patients prior to or during surgical procedures.
However, the inventors have observed that conventional marking inks, including those containing CAS No. 314-13-6, tend to rapidly lose visibility in certain applications, such as when used to mark internal tissue.
Thus, the inventors have provided embodiments of additional compounds useful, for example, in marking inks.